Magnetic resonance may be used to analyze medical and/or chemical samples. Specifically, the diverse chemical constituents and/or the spatial distributions of such constituents of the sample may be analyzed through the application of the magnetic resonance phenomena. In general, the physical context of the invention is a NMR probe for nuclear magnetic resonance or magnetic resonance imaging. An idealized illustration is shown in FIG. 1.
A magnet 10 having bore 11 provides a main magnetic field. In order to control the magnetic field with precision in time and direction, there are provided magnetic field gradient coils (not shown). The gradient coils are driven by gradient power supplies 16, 18 and 20, respectively. Additionally, other shimming coils (not shown) and power supplies (not shown) may be required for compensating residual undesired spatial inhomogenities in the basic magnetic field. An object or fluid for analysis (hereinafter "sample") is placed within the magnetic field in bore 11; typically, the sample is placed in a sample space of an NMR probe (not shown) and the NMR probe is placed within the bore 11. The sample is subject to irradiation by RF power, such that the RF magnetic field is aligned in a desired orthogonal relationship with the magnetic field in the interior of bore 11. This is accomplished through a transmitter coil 12 in the interior of bore 11. Resonant signals are induced in a receiver coil, proximate the sample within bore 11. The transmitter and receiver coils may be the identical structure, or separate structures.
As shown in FIG. 1, RF power is provided from transmitter 24, and is amplified by an amplifier 31 and then directed via multiplexer 27 to the RF transmitter coil 12 located within the bore 11. The transmitter 24 may be modulated in amplitude or frequency or phase or combinations thereof, either upon generation or by a modulator 26. Transmitter and receiver coils are usually not concurrently used as such. The identical coil may be employed for both functions if so desired. Thus, a multiplexer 27 is provided to isolate the receiver from the transmitter. In the case of separate transmitter and receiver coils, element 27, while not precisely a multiplexer, will perform a similar isolation function to control receiver operation.
The modulator 26 is controlled by pulse programmer 29 to provide RF pulses of desired amplitude, duration and phase relative to the RF carrier at preselected time intervals. The pulse programmer may have hardware and/or software attributes. The pulse programmer also controls the gradient power supplies 16, 18 and 20, if such gradients are required. These gradient power supplies may maintain selected static gradients in the respective gradient coils if so desired.
The transient nuclear resonance waveform is processed by receiver 28 and further resolved in phase quadrature through phase detector 30. The phase resolved time domain signals from phase detector 30 are presented to Fourier transformer 32 for transformation to the frequency domain in accordance with specific requirements of the processing. Conversion of the analog resonance signal to digital form is commonly carried out on the phase resolved signals through analog to digital converter ("ADC") structures which may be regarded as a component of phase detector 30 for convenience.
It is understood that these resolved data signals from the phase detector 30 may be directly stored in a storage unit 34. The Fourier transformer 32 may, in practice, act upon a stored (in storage unit 34) representation of the phase resolved data. This reflects the common practice of averaging a number of time domain phase resolved waveforms to enhance the signal to-noise ratio. The transformation function is then applied to the resultant averaged waveform. Display device 36 operates on the acquired data to present same for inspection. Controller 38, most often comprising one or more computers, controls and correlates the operation of the entire apparatus.
In conducting NMR experiments, the coil 12 must be tuned to the resonant frequency of the nuclei to be observed. Additionally, the impedance of the coil 12 should be electrically matched to the impedance of the transmission line 19 which is optimally coupled through the multiplexer 27 to the receiver 28 to obtain the maximum transfer of energy and to obtain the best signal to noise ratio (SNR). To tune and match the coil 12, conventional NMR coils have variable capacitors. Typically, at least one variable capacitor is adjusted to tune the coil to the desired resonant frequency and at least another variable capacitor is adjusted to match the impedance of the coil. To adjust the capacitance of the variable capacitors, mechanical linkages are coupled to variable capacitors in the coil.
The probe is a critical component in NMR data acquisition. Among other functions, the NMR probe provides mechanical support for the sample and coil, and the NMR probe provides electrical connections between the coil and the NMR apparatus. The NMR probe is placed into the bore 11 to position the sample and coil in a preselected position along the center of the bore 11. FIG. 2 illustrates the mechanical structure of one example of a contemporary NMR probe 50. Briefly, the NMR probe 50 includes a box 52, three tuning rods 54a, 54b and 54c and a pair of board levels 56a and 56b. The coil (not shown) is located above the board level 56a e.g; axially beyond board level 56a. The sample is placed within the interior volume defined by the coil and typically in the center of the coil. For example, the coil may be a simple LC circuit with variable capacitors connected to the coil (not shown). The variable capacitors are typically located on the opposite side of the board level 56a as the coil.
To adjust the capacitances of the variable capacitors, the tuning rods 54a, 54b and 54c each comprise an assembly of concentric rods 58a and 58b. The concentric rods 58a and 58b are mechanical linkages that are coupled to the variable capacitors. The inner rod 58a rotates to adjust one of the variable capacitors on the board level 56a, and the outer rod 58b rotates to adjust another variable capacitor on the board level 56a. The box 52 supports the tuning rods 54a, 54b and 54c and the board levels 56a and 56b. Additionally, the box 52 houses connectors to the NMR probe that link the coil to the NMR apparatus described above. Furthermore, an outside shield tube (not shown) surrounds the tuning rods 54a, 54b and 54c, the board levels 56a and 56b, the variable capacitors and the coil.
When NMR experiments are performed, the box 52 is positioned outside the bore 11 the of magnet 10, and the board levels 56a and 56b are within the bore 11. To tune and match the coil for the NMR experiment, an operator manually rotates the mechanical nubs 60 associated with each concentric rod at the base of each tuning rod. Rotating the mechanical nubs 60 rotates the respective concentric rod of the tuning rod which adjusts the capacitance of the associated variable capacitor.
One shortcoming of the contemporary NMR probe is that the manual adjustment of the mechanical nubs 60 is inconvenient and inefficient. Because the NMR probe 50 is positioned largely within the bore 11 for experiments, the mechanical nubs 60 need to be adjusted by hand at the bore 11 away from the control console and display 36 of the NMR apparatus. The manual adjustment is also time consuming and troublesome.
Thus, it is desired to develop a NMR probe that may be tuned and matched remote from the probe. It is also desired to develop a NMR probe that may be efficiently and conveniently tuned and matched without interfering with the magnetic field of the NMR apparatus. Additionally, it is desired to develop a feedback system that enables automatic tuning and matching without aid from the operator.